Transmembrane Proton Transfer in Chloroplasts In Silico: pH Homeostasis of Lumen, Stroma, and Cytosol

Abstract

The research described in this paper is based on a mathematical model for the proton-dependent regulation of electron and proton transport in chloroplasts. We have considered redox transformations of photosynthetic (PS) reaction centres PS1 (P₇₀₀) and PS2 (P₆₈₀) and of the mobile electron transport carriers ferredoxin (Fd), plastoquinone (PQ), and plastocyanin (Pc), as well as the transmembrane proton transport processes associated with electron transport and ATP synthesis by membrane ATP synthase. The simulation results are in good agreement with the literature data on pH measurements in the lumen (pH_in) and stroma (pH_out) of chloroplasts at different cytosol pH values (pH_cyt). The steady-state pH values established in these compartments when chloroplasts are illuminated under photophosphorylation conditions satisfy the following inequalities: pH_in ≈ 6.2–6.4 < pH_cyt ≈ 7.2–7.4 < pH_out ≈ 7.8–8.0. Variation of the pH_cyt model parameter affects the electron flows along different electron transport pathways, such as non-cyclic electron transport from PS2 to PS1 and further into the Calvin–Benson cycle, cyclic electron transport around PS1, and pseudocyclic electron transport involving molecular oxygen (the ‘water–water’ cycle). It has also been shown that sufficiently strong acidification of the cytosol (pH_cyt < 7) reduces electron efflux from the acceptor side of PS1 (noncyclic and pseudocyclin electron transport) and stimulates cyclic electron transport around PS1, and also decreases the rate of pH-dependent ATP synthesis.